1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus and more particularly to a sheet processing apparatus and an image forming apparatus having the same, wherein the sheet processing apparatus folds a sheet bundle in two and deforms a fold end portion of the folded sheet bundle.
2. Description of the Related Art
A conventionally known sheet processing apparatus bundles multiple sheets having images formed thereon, staples the center of the sheets, folds the sheets, and shapes them into a booklet-shaped sheet bundle (hereafter also referred to as a booklet). The sheet processing apparatus overlays sheets having images formed thereon one by one to prepare a sheet bundle. The sheet processing apparatus then staples the center of the sheet bundle and then folds the sheet bundle by pressing the sheet bundle into a nip of folding rollers using a pressing plate.
When a sheet bundle contains many sheets (e.g., 20 sheets or more), however, the fold end portion increasingly bends from the inside of the booklet to the outside (cover) and may not be folded fully. Such a booklet, when folded, may be half opened loosely and may not be placed flat. It is difficult to stack many booklets. The sheet processing apparatus has been requested to prevent the fold end portion of a booklet from bending and to fully fold booklets.
To solve this problem, U.S. Pat. No. 6,692,208 discusses a sheet processing apparatus that squares the fold end portion of a booklet to prevent the fold end portion from bending. The sheet processing apparatus discussed in U.S. Pat. No. 6,692,208 conveys a booklet with a fold end portion thereof being the front in a conveying direction. A holding unit nips to fasten the vicinity of the fold end portion. A pressing roller presses the fold end portion projecting from the holding unit to square the fold end portion.
The sheet processing apparatus discussed in U.S. Pat. No. 6,692,208 presses the booklet fold end portion projecting from the holding unit. If a large part of the fold end portion projects, the deformed fold end portion may bulge outward at both edges and leave a press mark on both edges due to the holding unit. Such a press mark is undesirable on booklets for a cosmetic reason.
To prevent the press mark from being formed, for example, a booklet may be nipped and fastened so that the fold end portion is positioned in between the holding unit and does not project from the holding unit. The pressing roller may be inserted in between the holding unit to press the fold end portion.
However, nipping to fasten the booklet presses the booklet in a thickness direction and bulges the fold end portion. For example, normally inserting the pressing roller increases the amount of the pressed (collapsed) fold end portion when the booklet fold end portion projects toward the pressing roller. As a result, the pressing roller may deform unintended sheets (e.g., those at the center) of the booklet and degrade the aesthetic quality at the fold end portion. Decreasing the amount of pressing roller insertion may prevent the amount of the pressed (collapsed) fold end portion from increasing. However, the fold end portion may not be fully pressed if a small amount of the fold end portion is deformed. The tip position of the fold end portion varies with the amount of the fold end portion formed by the holding unit. The pressing roller causes differences in the amount of pressed (collapsed) fold end portions. As a result, fold end portions are inconsistently shaped and the fold end portion quality may degrade.
The present invention provides a sheet processing apparatus and an image forming apparatus having the same, wherein the sheet processing apparatus can prevent a fold end portion from bending and stably form the fold.